can anyone explain...?

[worms]

This afternoon I had a look at an old rubble-stone garden wall that I was repointing last summer. Since there has been quite a deep frost, I wanted to check that the new pointing was still OK. On certain sections of the new pointing, there is surface ice, possible a couple of mm thick, but definitely ice rather than frost. There is no ice on any of the old pointing or any of the stones. It is not on the most recent sections of pointing (done in September) but on the bits done in July/August. It is not on the sections where the mix was a bit wetter and sloppier or the bit that was done in the rain when the mix on the board just got wetter and wetter (but I was determined to finish).

So what is going on here – is this moisture from the wall, or condensation forming on warmer sections of the wall or what? And since the mortar should be relatively absorbent and porous, how is it possible for water to stay on the (vertical) surface long enough to become ice? Or is this just one of life’s mysteries (like “star snot” : http://www.bbc.co.uk/scotland/outdoors/articles/jelly/)

Or do I need to get out more?

 

[Penners]

Well, here's a wildly improbable theory.

Presumably the newest pointing is still very white in colour, while the older stuff has weathered down? (If that's not the case, then I shouldn't bother to read further!).

It's known that a dark-coloured surface will absorb more heat than a light-coloured one. Perhaps the older, more weathered stuff absorbed sufficiently more heat to make it less likely to support the ice covering. i.e. older-warmer, newer-colder.

Fire away....

 

[middi]

It all sounds a bit odd to me, Penners theory is very accurate in principle but probably not applicable in a sunless country

Are you sure a dog ( or human) hasn't cocked his leg against the wall just prior to the frost, if that is the case Penners theory will hold.

 

[worms]

Thanks for the suggestions. At three to four feet up, it would need to be a giraffe rather than a dog, cocking its leg (unlikely to be human in this location). The pointing is all a fairly uniform colour and not that white (I used sharp sand from B&Q which is quite red in colour). This bit of wall is unlikely to see any sunlight until about May.

The wall has a higher ground level behind it but the ice is above that ground level where water could possibly have been seeping through. I think it must be something to do with relative temperature in the wall itself - perhaps lower down the ground behind is keeping the wall slightly warmer and nearer the top the wall reaches ambient quicker, leaving a level where condensation can form and freeze. Still doesn't explain why it is only on particular bits, but that could relate to the density of the infill and possible air spaces in the wall itself. Time will tell whether these bits of pointing fall out when it all thaws!

(I would try to post up a photo, but it would expose the poor quality of the job!)

 

[Penners]

At three to four feet up, it would need to be a giraffe rather than a dog, cocking its leg

Once you have dismissed all the impossibilities, what remains - however improbable - must be the truth. :wink:

Actually I think your theory about the height of the ice sounds very plausible. Dull, though!

 

[worms]

Well, maybe the giraffes are coming in unnoticed at night, but the treacle on the gateposts seems to keep the elephants out!

 

[Penners]

Ah yes. An old recipe, but efficacious nonetheless. And one of my favourites.

I haven't seen an elephant around here for... ooooh.... ages.